1. OBJECTIVE - To determine and the thermal conductivities of various metals and compare them with the ideal values - To

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1. OBJECTIVE - To determine and the thermal conductivities of various metals and compare them with the ideal values - To compare the thermal conductivities and thermal resistances of various metals at different lengths 2. THEORY Heat Conduction occurs whenever there is material contact and temperature differences between substances (atoms and molecules). This is the case in solid, liquid and gaseous substances. It also applies when there is a bodily contact between two surfaces, such as a pot on a stove. The thermal conductivity for solids is quite high, but it is quite low for liquids and gases. A thin gas layer is therefore used by double-glazed windows as thermal insulation. Cooling fins use the good thermal conductivity of metals to distribute the heat over the entire length. Thermal conductivity and Heat flux Thermal conductivity, λ is the characteristic variable of the material for good or for poor heat conduction. The higher the thermal conductivity of a material, the lower is the temperature difference at the same heat flux. The heat flux can be calculated using the following formula; Q˙cond =λ⋅LA⋅(T1−T2) Where; Q= heat flux in Watts λ= thermal conductivity in W/m2−K A= cross sectional area in m2 T= temperaturne in K Table 6.1 Thermal conductivity of various materiais T2=42.
Thermal Resistance The thermal resistance indicates how well the heat is dissipating. This is practically unimpeded when there is very low thermal resistance. The temperature difference over the thermal resistance is very low. If there is a high thermal resistance, heat dissipation is impeded. The temperature difference over thermal resistance is high. In this case, even a low heat flux will result in a high temperature difference. For thermal resistance which results from heat conduction, the following equation is being used; R=λ⋅AL=QT1−T2 Where; R is the thermal resistance in K/W is the thermal conductivity in W/m−K A is the cross-sectional area in m2 L is the length of the heat-conducting layer in m T is the temperature in K 3. EQUIPMENT WL420 Heat Conduction in Metals
PROCEDURE Calibration of the device i. Select "Calibration" in the "Test" tab. ii. Position the heater in the uppermost position without a sample so that the heat can be dissipated in the environment via the insulation. (The heater is locked into the guide without a sample so that no heat is dissipated to the underside by heat conduction). iii. Set the heat flux to 3 W by increasing the w(P) percentage. iv. Observe the temperature TH until the steady state is reached. v. If the steady state is reached, press the calibration button to store the correction factor for calculating heat loss. Conducting the experiment i. First the selected sample is placed in the measurement set-up. ii. Enter the length sample. iii. Set the heat flux at 15 W and cooling power at 80%. iv. The measured values are observed in the time graph until the steady state is reached. v. If needed, heat flux and cooling power can be varied. vi. If the steady state is reached, the measuring point can be recorded. vii. Repeat the steps for different lengths of different metals.
Lab Worksheet Guidelines 5. DATA AND RECII TE
Lab Worksheet Guidelines Guide Questions for Lab Memo 1. Explain: a. Heat Conduction in Metals b. Thermal Conductivity c. Thermal Resistance 2. Describe the relationship between: → a. Material vs Thermal Conductivity (both lengths) H b. Material vs Thermal Resistance (both lengths) 3. Compare the thermal conductivities of various metals with the ideal values 4. Discuss accuracy of the values. rationgy -leslmel coidudivity T1T2 lering Hine take